1. Field of the Invention
The present invention relates to a piezoelectric vibrator in which thickness-shear vibrations are excited and, more particularly, to a vibrator wherein the relative vibration level of unnecessary vibrations is damped more than that of a desired vibration. The invention also relates to damping of the relative vibration level of unnecessary vibrations of the piezoelectric vibrator.
2. Description of the Related Art
Crystal vibrators generally have a high Q, which represents the good sharpness of resonance, and, using thickness-shear vibration, have a flat frequency-temperature characteristic at and around a room ambient. As a result, crystal vibrators have been widely used as a resonator for an oscillator and as a filter of a communication equipment and tool.
Crystal vibrators can be categorized into the single mode type which is used as an element of a filter, an oscillator and the like, and the multiple mode type which is used as an element of a filter (a monolithic crystal filter, for example) and the like.
A case wherein the conventional crystal vibrator of the multiple mode type is used as a element for a filter will now be described, with reference to FIGS. 1, 2A, and 2B. FIG. 2B is a sectional view taken along a line a--a in FIG. 2A. Hatched portion in FIG. 2A does not show a sectional view but the positional relation of plural electrodes which overlapped another. The same can be said of other figures.
Quartz crystal plate 1 is of AT-cut, as roughly shown in FIG. 1. The AT-cut is defined as cutting a quartz crystal along the plane x - z of crystal axes (x, y, z) of the quartz crystal rotated round axis x in the direction of y to z by an angle of about 35.degree.15'. Rotated coordinate axes are defined as y' and z' axes as shown in FIG. 1. Although quartz crystal plate 1 is shown as a rectangular one in FIG. 1, it is usually formed as a circular disk, for example, as shown in FIGS. 2A and 2B.
Two couples of paired electrodes 2 and 3 are formed on major surfaces of crystal plate 1. Two couples of paired electrodes 2 and 3 comprise separated electrodes (or two electrodes in other words) 4, 5 formed on one major surface of crystal plate 1 and common electrode 6 formed on the other major surface thereof. Separated electrode 4 and common electrode 6 face each other and form one paired electrode 2. Separated electrode 5 and common electrode 6 face each other and form the other paired electrode 3. Tab electrodes 7 and 8 are extended from separated electrodes 4 and 5 to both ends of crystal plate 1. A tab electrode 9 is also extended from common electrode 6 to another end of crystal plate 1.
The crystal vibrator uses paired electrodes 2 as input terminals and paired electrodes 3 as output terminals, for example, and it serves to output those signals of input signals which are in a necessary frequency band.
In the case of the crystal vibrator having the above-described arrangement, thickness-shear vibrations are excited mainly at resonance (or excitation) region 10 of crystal plate 1 which comprises a portion among separated electrodes 4, 5, common electrode 6 and electrodes 4, 5, 6. The thickness-shear vibrations represent those which are reversely shifted one another on the major surfaces in the direction of axis x. The thickness-shear vibrations have various kinds of vibration mode which are represented by mode symbols (y, x, z). In the case of the crystal vibrator having such arrangement as shown in FIGS. 2A and 2B, separated electrodes 4 and 5 act as single electrode 11 to vibration fs of the symmetrical mode (FIG. 3A) while they act independently of the other to excite vibration fa of the anti-symmetrical mode (FIG. 3B). Vibration fs0 of (y, 1, 1) mode is shown an example in FIG. 3A and vibration fa0 of (y, 1, 2) mode is shown in FIG. 3B. z of mode symbols (y, x, z) is occupied by an even number in the case of symmetrical vibration fs and it is occupied by an odd number in the case of anti-symmetrical vibration fa.
A filter element is usually formed regarding vibration of (y, 1, 1) mode in symmetrical vibration fs as main one fs0 and vibration of (y, 1, 2) mode in anti-symmetrical vibration fa as main one fa0. As shown by a reactance characteristic curve in FIG. 4A for example, antiresonance frequency fx of main vibration fs0 in symmetrical vibration fs is made consistent with resonance frequency fy of main vibration fa0 in anti-symmetrical vibration fa to obtain such a transmission characteristic at the desired band as shown in FIG. 4B. The symbol f0 represents the center frequency of the passband of a filter.
y of mode symbols (y, x, z) represents the number of half waves in the direction of thickness (or axis y'), that is, the order of overtones. x and z of mode symbols (y, x, z) denote numbers of antinodes of vibrations which are shifted in the directions of axes x and z' at their resonance regions. The vibration fs0 of (y, 1, 1) mode in symmetrical vibration fs, for example, exhibits the maximum shift distribution in the center of the resonance region and a shift distribution like a cosine wave in the directions of axes x and z' in the resonance region. The vibration of (y, 1, 2) mode in anti-symmetrical vibration fa exhibits a shift having two antinodes of vibration in the direction of axis z' and one antinode of vibration in the direction of axis x in the resonance region.
Various kinds of unnecessary vibration fp (which will be hereinafter referred to as unnecessary vibrations) are excited in addition to main vibration fs0 of (y, 1, 1) mode and main vibration fa0 of (y, 1, 2) mode, which are necessary to obtain the transmission characteristic, in the case of the vibrator of the multiple mode type. Vibration fsl of (y, 1, 3) mode and vibration fs2 of (y, 3, 1) mode, for example, are unnecessary vibrations fp of the symmetrical mode (see FIG. 5A). Similarly, vibration fa1 of (y, 3, 2) mode and vibration fa2 of (y, 1, 4) mode are unnecessary vibrations fp of the anti-symmetrical mode (see FIG. 5B). Unnecessary vibrations fp are inharmonic to each other and they are in an inharmonic relation to main vibrations fs0 and fa0. Therefore, they are called inharmonic or unwanted vibrations.
As shown in FIG. 6, these unnecessary vibrations fp generates unnecessary pass bands outside the desired pass band at which the transmission characteristic of the filter is obtained.
Drawbacks caused by the presence of the unnecessary vibrations can be seen in the crystal vibrator of the single mode. When a filter element of the lattice type is formed using two crystal vibrators of the single mode, for example, the unnecessary pass bands are also created outside the desired pass band, deteriorating a characteristics of stop band similarly to the case shown in FIG. 6. When the crystal vibrator is used as an element of an oscillator, S/N ratio of signals oscillated becomes worse.
There are well known the following measures for solving problems caused by the unnecessary vibrations. (1) The diameter and thickness of electrodes are made small to reduce the amount of quasi plate-back (or amount of frequency lowered by the formation of these electrodes), in order not to trap vibrations except the main ones in the resonance region. (2) Bonding material 12 and the like are applied as an energy dissipating mass load to crystal plate 1, as shown in FIG. 7, to mechanically restrain the unnecessary vibrations and damp the relative vibration level of the unnecessary vibrations.
When the diameter of electrodes is made small, however, Q value is decreased, insertion loss becomes large and the characteristic of cutoff frequency in the pass band of a filter is deteriorated. The equivalent inductance due to electrodes becomes large and maximum width of the pass band is limitted, thereby causing design freedom to be limited. When the thickness of electrodes is made small, it becomes difficult to obtain a good energy trapping effect. When additional mass load is added to the crystal plate, Q value is decreased and manufacturing workability is lowered much.